1. Field of the Invention
The present invention relates to a process for the hydrogenation of hydroformylation mixtures from the preparation of higher oxo alcohols by hydroformylation of the corresponding olefins.
2. Description of the Background
Higher alcohols, in particular those having from 6 to 25 carbon atoms, can be prepared, as is known, by catalytic hydroformylation (or oxo reaction) of the olefins having one carbon atom less and subsequent catalytic hydrogenation of the aldehyde- and alcohol-containing reaction mixtures. They are predominantly used as starting materials for preparing plasticizers or detergents.
It is known that, in the catalytic hydroformylation of olefins, reaction mixtures are formed which, apart from the desired products, i.e. aldehydes and the corresponding alcohols, depending on the catalyst and the reaction conditions, can comprise by-products and secondary products, such as unreacted olefins, saturated hydrocarbons formed from the olefins by hydrogenation, water, esters of the desired alcohols (e.g. formates), acetals of the target products aldehyde and alcohol, enol ethers and other by-products or secondary products. These substances can be subdivided into low-boilers having a boiling point below the boiling point of the aldehyde and high-boilers having a boiling point above the boiling point of the alcohol. In the hydrogenation of the reaction mixtures, from some of the by-products, such as esters and acetals, the alcohols wanted as target product are formed, which improves the yield. In particular it is desired that the formates, which can occur in amounts up to 10% by weight, are hydrogenated under comparatively mild conditions and particularly at low pressure using commercially conventional catalysts to give the desired alcohol (and methanol as by-product).
The catalytic hydrogenation of reaction mixtures which were prepared by cobalt-catalyzed hydroformylation of olefins having from 2 to 24 carbon atoms is described, for example, in DE 35 42 595. The hydrogenation is carried out in two stages. In the first stage, the hydroformylation mixture is hydrogenated at 150-230.degree. C. and a hydrogen pressure of 10-350 bar with 80-95% conversion on a supported SiO.sub.2 catalyst which comprises 5-15% by weight of nickel and 2-20% by weight of molybdenum in the form of molybdenum oxide. In the second stage, the hydrogenation is completed at 150-230.degree. C. and 10-350 bar hydrogen pressure on a catalyst whose active mass consists of 55-60% by weight of cobalt, 15-20% by weight of copper, 4-10% by weight of manganese and 2-5% by weight of molybdenum in the form of molybdenum oxide and, if appropriate, up to 10% by weight of activating additives. In the process; the formates and acetals present in the mixture are converted to the corresponding alcohols. However, the process has the disadvantage that the hydrogenation is carried out in two stages and at high pressures--in the example at 250 and 245 bar.
According to U.S. Pat. No. 5,399,793, for the hydrogenation of cobalt-depleted reaction mixtures, as arise in the hydroformylation of C.sub.5 -C.sub.12 olefins, Ni/Mo catalysts on Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3.SiO.sub.2 as support materials are used. The complete process comprises the following individual steps:
(a) cobalt-catalyzed hydroformylation, PA1 (b) cobalt depletion of the reaction mixture, PA1 (c) hydrogenation of the crude reaction mixture at elevated temperature and at elevated pressure, PA1 (d) production of alcohols having very low amounts of aldehydes by distillation, and PA1 (e) finish-hydrogenation of the alcohols.
The hydrogenation of stages (c) and/or (e) can be carried out using a bimetallic, phosphorus-free Ni/Mo hydrogenation catalyst. This hydrogenation catalyst produces fewer high-boiling by-products than a corresponding phosphorus-containing catalyst. A disadvantage with this process is that to prepare an on-specification alcohol which is suitable for preparing plasticizers, two hydrogenation stages are necessary and that at least in the hydrogenation stage (b) a relatively high pressure of 1000 psig (about 70 bar) is necessary.